This invention relates to a process for the preparation of bismuth telluride and bismuth telluride composite thermoelectric materials. More particularly, this invention is in a method for the molecular chemical preparation of bismuth telluride and bismuth telluride composite thermoelectric materials by a chemical process.
It has been found that chlorofluorocarbon (CFC) refrigerants are detrimental to the environment and therefore a planned phase-out of CFC refrigerants may be imminent. As a result, considerable interest has developed in alternative refrigeration and cooling technologies. It is possible that alternatives to CFC liquid-gas expansion systems such as solid-state thermoelectric devices might readily be developed into a substantial technology in the near future.
Thermoelectric cooling devices based upon the Peltier effect have been used for many years in specialized applications. At the present time, these units do not achieve the performance currently available with CFCs. Typically, CFC systems operate near 40% of Carnot efficiency while the best thermoelectric systems reach only about 10% of Carnot efficiency (Vining, C. B., Proceedings of the 1992 1st National Thermogenic Cooler Conference, Fort Belvoir, Va., Sep. 17, 1992). Poor performance of the thermoelectric materials is a significant contributing factor to the low efficiency.
Certain semiconductor materials, particularly bismuth telluride-based alloys, are the materials of choice in modern thermoelectric coolers. These alloys are commonly made through metallurgical melt processing, i.e. by co-melting appropriate amounts of the pure elements in sealed vessels at temperatures above 600.degree. C., mixing, and then subjecting the melts to controlled cooling (F. D. Rosi, B. Abeles and R. V. Jensen, "Materials for Thermoelectric Refrigeration", J. Phys. Chem. Solids, Pergamon Press 1959, Vol. 10, pp. 191-200). This batch processing approach is both equipment and labor intensive, while thermoelectric elements cut from the solidified alloys tend to be somewhat fragile. An alternative approach, amenable to automated production, is the fabrication of thermoelectric elements from polycrystalline powders (Goldsmid, H. J. Thermoelectric Refrigeration, Plenum Press, New York, 1964, p. 198; Schreiner, H. and Wendler, F., Z. Metallk, 1961, 52, 218). These powders are commonly obtained by comminuting solidified melts and classifying the resultant particulate material. Polycrystalline thermoelectric elements offer improved structural integrity, although they generally exhibit some degradation in thermoelectric performance due to the anisotropic nature of the material. Alloying is done to optimize thermoelectric performance, and is limited to elements in Groups IV, V and VI in the periodic table because it is dependent upon the wide-range, solid-solid solution properties of the components.
The method for evaluating the performance of a thermoelectric material is the determination of its figure-of-merit, Z. The figure-of-merit is defined as, Z=S.sup.2 /.rho.K, where S is the thermopower (Seebeck coefficient), .rho., the electrical resistivity and K, the thermal conductivity (Goldsmid, H. J., "Thermoelectric Refrigeration" Plenum Press, New York 1964, p. 11)
Early work by Rosi, Abeles and Jensen, supra. and W. M. Yim and F. D. Rosi ("Compound Tellurides and Their Alloys for Peltier Cooling--A Review", Solid State Electronics, Pergamon Press, Vol 15, 1972, pp. 1121-1138) suggests that for semiconductors, arrays of dislocations at phase boundaries, submicroscopic precipitates, and distortions in the lattice structure would be effective in scattering phonons, but ineffective in scattering electrons which have relatively longer wavelengths. A recent report indicates a 15 to 30% reduction in the thermal conductivity of SiGe alloys through the inclusion of nanosized, BN and Si.sub.3 N.sub.4 particles ("Phonon Scattering by Ultrafine Particulates in SiGe Alloys at High Temperatures", J. W. Vandersande, J. P. Fleurial, C. B. Vining, J. Beaty, J. Rolfe and P. Klemens, Springer Series in Solid-State Sciences, Vol. 112, Springer Verlag, Berlin, 1993, pp. 44-45). This selective scattering phenomenon could be used to improve the figure-of-merit for thermoelectric materials by decreasing their thermal conductivities, K, while leaving their electrical resistivities, .rho., essentially unchanged.
The background technology recited above from the cited publications is incorporated herein by reference.